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9601 SE 36th Street
Mercer Island, WA 98040
Phone: 206.275.7608
Fax: 206.275.7814
Email: publicworks@mercer. . .
Hours: 7:30 AM - 4:00 PM (Mon-Fri)
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Traffic Signals


Mercer Island is a built out community with the majority of growth in recent years in the town center. Today, we have more cars, more pedestrians, and more bicyclists using our streets than ever before. Building new roads to handle the increased traffic volumes isn't a realistic solution in our "built" environment; effectively managing our streets is.  Every signalized intersection on Mercer Island addresses a variety of needs.  The signal must be effective and functional for pedestrians, automobiles, bicyclists, transit, and large trucks.  The simple fact is that not everyone can have a green light or a walk signal all the time.  For one group to have a green, another must have a red.
Why do we need traffic signals?
As traffic volumes increase on roads beyond the capacity of lesser intersection controls such as four-way stops, it may be necessary to install a traffic signal.  Traffic signals offer maximum control at intersections. They are designed to ensure a safe, orderly flow of traffic, increase the traffic handling capacity, provide safety for motorists or pedestrians while crossing a busy intersection, and help lessen the severity or frequency of collisions between vehicles entering intersections.
Where are they located on Mercer Island?
There are 13 signals on Mercer Island.  Most are located near the I-90 corridor and are owned and operated by the Washington State Department of Transportation.  The remainder are in the town center and the Island Crest Way corridor.  These are owned by the City but maintained by King County.  Refer to the map for specific locations.
Who is responsible for maintaining them?
It depends on the location.  In general, those in close proximity to I-90 are the responsibility of the Washington State Department of Transportation (WSDOT).  The others are the responsibility of the city but the maintenance is handled by King County signal crews by contract (see map).  Wherever it is, if the signal is dark, always treat the intersection as a four-way stop.
Who do I contact if a signal is dark or malfunctioning?
Please report malfunctioning traffic signals to Mercer Island’s Public Works Department at (206) 275-7608 during work hours, or to the City's 24/7 dispatch number (425) 577-5656 after-hours and on weekends.  If the signal is dark, always treat the intersection as an all-way stop.
Are they energy efficient?
All of the traffic signal lights (red, yellow, green) have been converted from incandescent bulbs to light emitting diodes (LED).  These are tiny, purely electronic lights that are energy efficient and have a long life.  LED's have replaced the old-style incandescent bulbs rated at 100 to 150 watts.  The LED displays have a 10 to 15 watt rating and generate very little heat, which cuts down on electricity demands.  Because traffic signals operate 24 hours a day, 365 days a year, the opportunity for energy and labor savings is significant.  LED’s provide three primary advantages over old-style incandescent bulbs:
  • LED's are brighter.
  • LED displays last for years, while incandescent bulbs last for approximately 12 months.
  • LED displays save a lot of energy and reduce operating costs.
How Do Traffic Signals Work?
There are two types of signals, fixed time and traffic responsive.  Fixed-time signals assign the green light to the different approaches of an intersection for a predetermined amount of time.  Some of them also can be set to change to different green times during peak traffic hours.  These signals can be found in urban areas where traffic movements are fairly predictable.  Traffic responsive signals change the lights according to the amount of traffic in each direction. These signals use sensors to detect the number of vehicles, and automatically adjust the length of the green light to allow as many vehicles as possible through an intersection before responding to vehicles on another approach.  This is the type on Mercer Island. 
A signal detection loop
A signal detection loop
What kinds of traffic sensors are used for traffic signals?

Signal detection loops and video detection cameras are the two types of traffic sensors used to operate traffic signals.  Cameras are now the most cost-effective way of performing vehicle detection. Previously, the primary method of vehicle detection was done by cutting the pavement a few inches deep and installing a wire "loop" just below the surface. This "loop" is charged with a small electric current originating from the traffic signal controller.  As a metal object (i.e. vehicle) travels through the electric current's field, the change in the inductance from the metallic object triggers an output that a vehicle is within the "loop".  However, there are some disadvantages with loops:
  • They have trouble identifying bicycles and motorcycles made with a mix of metallic and non-metallic materials or have the wrong induction signature. 
  • They require numerous junction boxes in the sidewalk.
  • Some people may consider them unsightly.
  • They require replacement when a roadway is repaved.
  • They require replacement when utility trenches and other construction cut the loops.
  • They have a shorter service life (5 to 10 years).
  • The cutting of the pavement to install the loops can also weaken the strength of the pavement resulting in greater maintenance cost for pavement repair.  When in-ground loops fail, the entire loop must be recut into the pavement again resulting in added cost and disruption to traffic.
Video detection cameras, like most electronics, have dropped in price and improved in function.  Installation is done above the surface of the road on the traffic signal pole.  When cameras fail, they are simply and quickly replaced without a great impact to traffic flow.  
The cameras are focused on vehicles and bicycles as they enter defined areas or "zones" within the camera's field of view sending a signal to the traffic signal's controller that says a vehicle is requesting green time for its direction.  The cameras are mounted high above intersections for a better angle and wider view of traffic.  The camera view is a fixed focus, fixed location image.  There is no zooming or panning of the cameras. The image is analyzed by the camera processor ONLY for the simple presence of vehicles and bicycles within defined areas.  The cameras are selected for durability and economy, not for high resolution.  They are good enough to see where a vehicle is located, but not good enough to see inside a vehicle.  The video is analyzed by the processor in real-time, with no recording of the image stream.  It has no surveillance capability.
Are traffic signals coordinated to reduce or eliminate the need to stop at red lights?
The majority of traffic signals are coordinated (timed), depending on the time of day, the location, and (when applicable) the primary direction of travel.  Coordinating traffic signals to facilitate traffic flow in both directions simultaneously is complicated.  Timing must be designed so those vehicles traveling in both directions arrive at the intersections at the same time.  When one considers traffic from the side streets, and that the intersections are not uniformly spaced, it becomes clear that there are no perfect solutions, especially when pedestrians also want to cross the intersection.  Traffic engineers examine these circumstances very carefully and attempt to design "best-fit" timing.
There are many factors that can affect the design of signal timing plans, including the:
  • Volume of traffic on the side streets.
  • Crossing time required for pedestrians.
  • Distance between traffic signals.
  • Speed limit on the main street.
  • Total overall traffic volume.
  • Volumes of turning vehicles.
  • Number of lanes available for each movement of traffic.
Additional factors that traffic personnel have no control over but impact the effectiveness of the timing plans include the:
  • Actual speed of traffic.
  • Acceleration patterns of motorists.
  • Variations in the volume of traffic throughout the day.
  • Frequency of police and fire vehicles overriding the signal timing for emergencies.
  • Frequency of pedestrians using the pedestrian signals to cross at the intersection.
Do any traffic signals not run in coordination with other traffic signals?
Yes, that type of operation is referred to as actuated or demand-responsive operation.  It reduces the amount of time a vehicle has to wait for a green light when there is no conflicting traffic present.  The decision to run a signal in a demand-responsive operation usually involves a number of factors including, but not limited to the following:
  • Distance to adjacent traffic signals.
  • Volume of traffic.
  • Availability of video detection.
  • Other odd conditions like the presence of unique traffic generators (eg. Schools, large church, etc) on the side street.
How do signals for pedestrians work?
When a pedestrian pushes the button on the signal, a message is sent from the button to the electronic signal controller that controls the traffic signal.  The button is like a light switch—after you have pushed it once, it is “on” and stays on until the “Walk” light appears.  Once the initial “Walk” time has passed, the “Flashing Don’t Walk” will appear.  This means that if you have not started to cross the road yet, you should not step off the curb.  HOWEVER, if you are already in the process of crossing the road, the average pedestrian will have enough time to cross the entire road safely.  Some intersections are also equipped with “countdown” pedestrian signals that display the exact number of seconds remaining before the pedestrian needs to be clear of the intersection.
Why do I have to wait so long to get a green light or a WALK signal?
There are a number of reasons why drivers and/or pedestrians will find themselves waiting. Generally, if you actually time it and find yourself waiting for more than 120 seconds, there may be a problem with the traffic signal.  When a traffic signal is running in coordination with other traffic signals, it operates in a timed cycle. The coordination plan will allow the side street green light to appear only at a certain time in the cycle when it is designed to minimize the disruption of traffic on the main street. If you arrive after that time in the cycle, you will have to wait through one more cycle until that time comes around again. Cycle lengths range from 50 seconds to as much as 120 seconds, depending on time of day and the location.

When a traffic signal is not running in coordination with other traffic signals and is set up for demand-responsive operation, the controller of the intersection will give the side street a green (or a Walk) as soon as there is an appropriate gap in main street traffic. If there are no gaps of sufficient length in main street traffic, the traffic signal will continue to display a green light for a pre-determined maximum time for the main street.

If the signal is running in demand-responsive or coordinated mode, you may still find yourself waiting at a red light for some time. When this occurs, look for pedestrians who may have activated the pedestrian signal or fire trucks that may be approaching from any other direction. Both of these situations can make motorists wait for an adequate time for them to safely travel through the intersection.

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